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oooh, that's really good to know! Since mine are just low tech and not terribly deep, I didn't have a way to measure that. The plants still looked healthy, just maybe slower growing? But this is really good to know for me since I'm setting up two deeper tanks now. Thanks!In case you are curious - i measure the different with par meter from dense floating plants and no floating plant - at 15 inches the par changed from 80 to 20